1. Field of the Invention
The present invention relates to the tempering of glass sheets, and is particularly suitable for the fabrication of shaped, tempered glass sheets. While the invention will be described in conjunction with apparatus where glass sheets are tempered after being shaped by a so-called vacuum forming process by which glass sheets in succession are first heated to a deformation temperature, then brought into pressurized engagement against a vacuum mold while heat softened to shape each sheet to its desired shape and then the sheets so shaped are subjected to a rapid cooling from an elevated temperature sufficient to insure the imposition of at least a partial temper or toughening or heat strengthening of the sheet, it will be understood that the present invention may be employed in any environment in which glass sheets are first heated to a temperature above the strain point and approaching the softening point and then are suddenly chilled in order to toughen or to impart at least a partial thermal temper to the glass sheets.
The process of tempering glass sheets imparts to the tempered glass a surface compression stress that completely encloses an interior zone stressed in tension. Since glass is notoriously weak in tension and strong in compression, tempered glass is stronger than untempered glass by virtue of the skin of surface compression stress that surrounds the interior stressed in tension. However, in case the glass sheet being treated has flaws or in case the tension stresses temporarily established during the cooling operation become sufficient to overcome the inherent strength of the glass, the flawed glass may develop a tension stress in its surface of such a nature that during fabrication, and particularly, during the rapid cooling or quenching that follows the heating step, the glass fractures into extremely tiny fragments.
Unless the fragments are removed sufficiently rapidly before the next glass sheet in a series of glass sheets being fabricated under mass production conditions can be removed, the subsequent glass sheets are likely to develop scratches and other problems such as a jam-up, which results when a succeeding glass sheet comes into contact with particles that remain from a previous glass sheet that has been fractured. The succeeding glass sheet may also break if the fragments are not removed in a timely manner or, the next glass sheet in a series may have its surface marred by the particles or fragments of a previous glass sheet that have not been removed from the cooling station.
When glass sheets are supported in a horizontal plane at the cooling station, the space between the upper and lower nozzle boxes is usually so narrow that it is very difficult to remove fragments of fractured glass sheets from the cooling station in time to clear the station for the next successive glass sheet. It has been customary for operators to insert implements, such as wooden sticks, between upper and lower nozzles to force the fragments out of the space between the upper and lower nozzles. Manual manipulation of such implements to remove glass fragments is both expensive and time-consuming, even when fragments of flat glass are to be removed. Furthermore, the inability of implements to remove all of the fragments from the cooling station before the next sheet arrives has caused one or more successive sheets after the fractured sheet to be rejected because the remaining fragments either cause successive glass sheets to fracture or to become scratched so that they did not have the optical properties necessary for customer acceptance. In addition, when glass sheets are shaped prior to their being tempered they are usually transported in a direction parallel to the axis of bending so it becomes extremely difficult for operators with access to the space between upper and lower nozzles only from the sides of the cooling station to insert a tool or implement to remove the glass fragments unless the glass is shaped to a very shallow curvature of substantially uniform radius of curvature and the implements used to remove the glass fragments are shaped accordingly. Such problems of glass fragment removal does not exist when glass sheets are disposed in a vertical plane between opposed sets of nozzles also disposed vertically.
When glass sheets are transported by rotating driving discs that engage one edge of a series of glass sheets supported in an oblique plane, lowering a number of supporting discs along a length equal to a glass sheet length has helped to cause glass fragments to be removed by sliding engagement over a lower bed of tempering nozzles disposed in an oblique orientation. Other devices to improve the speed of removal of glass sheet fragments have involved the pivoting of the lower nozzle housing to an oblique orientation, a simultaneous pivoting upward of the upper nozzle housing simultaneously with reducing the pressure by which the tempering fluid is applied to enable an operator to have more ready access to remove glass fragments with wooden implements. Still another invention involves pivoting the lower nozzle box from a substantially horizontal orientation to an oblique orientation to permit glass fragments to slide downward away from the furnace exit whenever breakage occurs. Still another apparatus in the prior art that conveys glass sheets on a series of conveyor rolls uses means for pivoting both the upper and lower nozzle housings to oblique orientations to provide both additional access space for operators to remove glass fragments with hand operated tools and also to permit glass particles or fragments to slide below a level of support provided by the upper surface of the series of conveyor rolls of a roller hearth along which glass sheets are transported through a cooling station.
While some of the latter improvements provided benefits compared to those obtained merely by using hand implements to remove fragments whenever they occurred, sometimes there still remained some fragments after conclusion of a tempering operation.
2. Description of the Prior Art
U.S. Pat. No. 3,223,252 to Mikus supports and engages the lower end edges of a series of glass sheets supported on an oblique plane of support by a gas bed by rotatable driving discs that engage the lower edge of the glass to propel the sheets forward along the gas bed through both a heating station and a cooling station. Whenever a glass sheet breaks into fragments, a series of consecutive discs occupying a space at least equal to or slightly longer than the length of the supported lower end edge is removed from glass supporting position to allow the fragments to float downward unimpeded by the driving discs to one side of the bed. The series of discs is returned to the driving position in time to engage a succeeding sheet being treated.
U.S. Pat. No. 3,846,106 to Seymour discloses glass sheet quenching apparatus comprising upper and lower nozzle housings with a lower nozzle housing being pivotably supported by lifting and lowering means to enable the lower nozzle housing to pivot between a substantially horizontal orientation closely adjacent the lower surface of a heated glass sheet and a second position wherein the upper surface of the lower nozzle housing occupies an oblique orientation to facilitate removal of glass fragments by sliding when a glass sheet is broken during the delivery of cool tempering medium thereagainst.
U.S. Pat. No. 4,076,511 to Fulton, Edmunds and Shields discloses apparatus for removal of glass fragments from the glass sheet tempering apparatus by damping air flow to the cooling chamber simultaneously with the opening or the pivoting upward of the upper nozzle housing to enable operators to obtain more clear access to remove the glass fragments at the cooling station.
U.S. Pat. No. 4,138,241 to McKelvey discloses apparatus for tempering glass sheets in which glass sheets are supported upon rolls through a cooling station where provision is made to pivot both the upper and lower sets of nozzles in-between adjacent conveyor rolls so as to move the nozzle housings between essentially horizontal orientations and oblique orientations. Fragments slide over the obliquely disposed lower set. The upper set is recessed upward to loosen jammed fragments that are not free to slide.
None of the cited patents provided means for free fall of glass fragments to a position below the cooling station where the fragments can be removed. It is essential in all of the prior art patents that the glass be removed either by sliding by gravity over an obliquely disposed lower nozzle housing or by use of hand tools. It remained for the present invention to provide additional improvement over those available from the prior art patents.